Flea bites can swell up. Typically this occurs during the delayed reaction, 12-24 hours after the flea feeds. The edema (swelling) can accompany a welt-like lesion called a papule.
Edema & Flea Bites
Shortly after a flea bites, the nearby affected skin begins to elevate. The slightly raised lesion is called a wheal. Swelling (edema) may accompany the early reaction.
12-24 hours after the bite, the wheal is supplanted by a papule, which is a small, itchy, welt-like lesion. Some individuals experience a rash with numerous tiny papules, called maculopapules. Redness and edema can occur with both of these lesion types. Occasionally, the papule’s center develops a vesicle (tiny fluid-filled sac).
In extreme cases, delayed reactions can become blisters (bullous reactions). Fluid accumulates under the skin, eventually combining and swelling into a large blister (bulla).
Inflammation from Scratching
Scratching a flea bite causes inflammation and increases the sensitivity of nerves in the area. While not necessarily swelling, the skin can become inflamed.
Scratching a bite can also break open the skin and release fluid. This can trigger secondary bacterial infections. Infected bites can then fill with pus and resemble pimples.